Microsoft has made Windows 8.1 available in Windows Store as a free download for the Windows 8 users. Windows 8 users will see a notification in the Windows Store with a prompt to upgrade to Windows 8.1. The free updates comes with a lot of new features such as deeply integrated SkyDrive with placeholder files, enhanced Bing apps and Smart Search, re-sizable Start Screen tiles with some bundled apps.
Retail (boxed) copies of Windows 8.1 will be available for $119.99 for the "core" version and $199.99 for the Professional version.
If you are running Windows 8.1 Preview then keep it in mind, upgrading to 8.1 will remove all installed apps and settings but you can follow this guide for keeping your settings and apps intact. Upgrading from Windows 8 RTM to 8.1, on the other hand, saves all the settings.